The invention concerns a device for generating light, in particular light for use in length measurement using an interferometer. The frequency of the laser light emitted is controlled as a function of the wavelength in the surrounding gaseous medium (air) in such a way that the wave length in air remains constant.
With interferomic determination of displacement of a mobile component, it is of decided meaning to precisely know the wavelength of the employed laser light in the surrounding medium (henceforth called wavelength in air), since the displacement path exists in units of the wavelength in air. If one knows the exact frequency of the light wave beforehand, the possibility arises to determine the wavelength in air through an ascertainment of the refractive index.
The refractive index can be determined using the "parameter method"-the temperature, air pressure, and air moisture are measured and the index is formulmericly calculated. However, this parameter procedure has the disadvantage that important influences such as the composition of gas are not incorporated and thus eliminates possible operation of the interferometer in the workshop or open areas.
A device is already known where the wavelength in air is measured by an open etalon and the frequency of the light source is adjusted as a function of the refractive index of air, in such a way that the wave length in air for interferomic measurement remains constant. In this device, a Helium-Neon-Laser is employed as the light source. The variation of the emitted laser light's frequency is possible through adjustment of the laser resonator's length by means of warming and cooling or through utilization of the "Piezo-Effect."
Apart from a relatively high required power supply and size of a Helium-Neon-Laser a particular disadvantage appears that the range of frequency adjustability merely suffices for determining fluctuations in the refractive index in the magnitude of .DELTA.n.apprxeq.2.10.sub.-6, while in practice the resulting fluctuations are far higher, namely in the range .DELTA.n.apprxeq.10.sub.-4.
The object of the invention is to provide a device for generating light, in particular light for use in length measurement using an interferometer, that is able to hold the wavelength constant in the surrounding medium through steady changes in frequency with the existing fluctuations in the refractive index (magnitude of .DELTA.n.apprxeq.10.sup.-4). In addition, the device should present a compact model, in particular a small light source and if need be, the possibility of battery operation.
According to the invention, this is accomplished by the fact that the light source is a laser diode and a device is provided external to the laser resonator, for feeding back to the resonator, at selected frequencies, light emitted by the laser diode.
With the feeding back of the laser light at selected frequencies according to the invention's device, it is possible, within certain limits, to "lock" the emissions-frequency of the laser diode's light to the preferred feedback frequency, when alterations of the wave length in air or the refractive index in the surrounding medium occur, in such a way that the wavelength in air actually remains constant, whereby by this procedure the otherwise typically unsteady (discontinuous) behavior of the emission-frequency of the laser diode as a function of the operative parameters (injection current, temperature) can be avoided within a certain "locking range."
With this "locking" of the laserdiode's emission-frequency within a range of control, the device utilizes the known effect that feedback light at selected frequencies amplifies the corresponding emission-frequency-mode of the laserdiode or represses the unwanted build-up of other modes, in such a way that it is even possible to "lock" the emitted light of the laser diode to each frequency that corresponds to the preferred feedback frequency from the frequency selective feedback device.
The frequency selectivity of a feed back device can then be obtained according to a preferred embodiment, by an etalon of given length which is operated either in transmission or reflection, the surrounding medium being present between the reflective surfaces of the etalon, whereby at least a part of the laser light being frequency-selected by the etalon is fed back to the laser resonator of the laser diode. With regard to said "locking effect" a narrow-banded feed back from an etalon operated in transmission is particularly advantageous. With this embodiment, said preferred feedback frequency from the etalon in the range of the emission-frequency of the laser diode varies automatically as a function of the refractive index in the surrounding medium in such a way, that with the illustrated effect, the emission-frequency of the laser diode adjusts itself so that the wave length in air remains constant.
If one knows the wavelength in air or the refractive index of the surrounding medium (e.g. through electronic analysis of interference-rings in an open air-wave length-determination etalon), it is in principle also possible to adjust the feedback frequency in the laser diode and with it the "locked" emission-frequency of the laser diode, by varying, as a function of the refractive index or the wavelength, (e.g. piezo-electrically or electro-optically) the length of a closed etalon of the feed back device or the distance of this etalon from the laser diode.
According to a favorable embodiment of the invention, it is desirable if an electronic control device is provided that controls the injection current and/or the temperature of the laser diode as a function of the wavelength in air or refraction index of the surrounding medium.
At the start of a measurement series, the controlling device is first used for determining a suitable initial emission-frequency of the laser diode by applying a defined injection current and/or temperature. Starting from this emission-frequency, a frequency control keeps the wavelength in air constant even if the environmental conditions (refractive index) change. The criteria for these initial selections are a possible low energy consumption for the maintenance of the chosen operative parameters, as well as a possibly important distance between two adjacent mode-jumps, in the emission-frequency-spectrum of the laser diode as a function of the injection current and temperature.
The determination of the existing wavelength or the refraction index in the surrounding medium, which is necessary for the electronic control, may be accomplished by an etalon, the surrounding medium being present between the mirrored surfaces of the etalon and whose interference pattern is electronically evaluated. According to the mentioned arrangement of initial conditions, the possibility arises with the electronic controlling device (actual control function) to control the operative parameters (injection current, temperature) of the laser diode as a function of the wavelength in air thereby assisting the "locking" of the laser diode's emission-frequency to the preferred frequency from the feedback device. Through this combination of optical feed back regulation on one side and electronic regulation of the operative parameters on the other side, it follows that the emission-frequency remains "locked" over a large "locking range" to the preferred feedback frequency.
In this case no high claims are placed upon the accuracy of electronic regulation, since emission-frequency, which is necessary to hold the wavelength in air constant; within the "locking range" is determined by the signal from the feedback device.
If the laser diode frequency, based on a relatively wide-banded feed back at selected frequencies (e.g. from a solid glass etalon of small thickness) is not "locked", or at least not sufficiently "locked" to a specific optical feedback frequency, the control of the emission-frequency of the laser diode is also possible exclusively by means of the electronic controlling device as a function of the wave length in air or the refraction index in the surrounding medium. Thus a high accuracy claim is placed upon the electronic regulating control device.
In this case, said optical feed back effect is realized with the goal that feedback light amplifies the actual oscillating frequency-mode and simultaneously represses the build-up of adjacent modes. In this manner, the typically unsteady behavior of the emission-frequency of laser diodes can be avoided within the required range of control. Such an unsteady behavior would severely disturb the function of the device according to the invention.
Particularly in the context with multi-mode laser diodes, a two-stage procedure is also possible: at first by means of an etalon, not necessarily filled with surrounding medium, perhaps a solid glass etalon, that is preferably thinner than the length of the laser diode's laser resonator, light is optically fed back at selected frequencies, for the amplification of the actual oscillating modes or the repressing of the adjacent mode build-up; in a second stage (from a longer etalon) light is then fed back at that frequency, that should lead to the "locking".
The feedback effect also depends on the positioning of the etalon or a mirrored surface of the feedback device from the laser diode's laser resonator. This can be employed in order to control the preferred fed back frequency by controlling the distance of the etalon or the mirrored surface from the laser diode.
Further advantages and details will be illustrated in the following description of the drawings.